In magnetic resonance imaging (MRI), a scanner system uses three different purpose magnetic fields to obtain an image: the B0 field, gradient fields, and radio frequency (RF) fields. The B0 field is the main magnetic field used by the MRI scanner, and is static in time and homogenous inside the volume of imaging. The B0 field determines the resonance frequency of the atoms depending on the gyromagnetic ratio of the atom. To obtain an image, resonance frequency of the object is spatially modulated by the gradient fields. Modulation may be used to perform particular imaging operations such as slice selection, phase encoding and frequency encoding; however, common purpose of the gradient fields are also used to discriminate different spatial locations by applying additional magnetic fields which has a certain spatial dependency. In conventional MRI scanners, there are three gradient coils used to encode three spatial dimensions: the x-gradient, the y-gradient and the z-gradient. During an imaging sequence, spatial encoding of the object should be changed as a function of time for imaging purposes; therefore gradient coils should be driven dynamically as a function of time and wideband current waveforms are necessary. Finally, the RF field is used to excite the nuclear magnetic spins.
MRI sequences are often designed with generic, idealized magnetic field conditions. Due to hardware imperfections and physical limitations, these ideal conditions are rarely achieved and the magnetic fields applied to a subject can deviate from expectations. The deviations may cause artifacts and distortions in the image; however, as long as the deviations are precisely measured, resulting artifacts and image distortions can be corrected.
Conventional Nuclear Magnetic Resonance (NMR) probes may be used to monitor both spatial and temporal dependency of the magnetic fields to estimate the correct k-space trajectory to be used in the image reconstruction. Although, NMR probes provide effective field monitoring capabilities, the use of NMR probes can be costly. Also, NMR probes are not able to measure the concomitant fields (i.e., fields in the x- and y-direction) simultaneously with the field in the z-direction; rather NMR probes measure each field separately. Although fields in the z-direction are much more important and effective in MRI, fields in the x-y direction also cause artifacts and can be corrected with image processing if known.